Long-time favourite Yeats continued on the slide in the betting for the Vodafone Derby amid rumours of the horse's well-being.

The unbeaten Aidan O'Brien-trained colt has drifted alarmingly, with one leading bookmaker claiming the son of Sadler's Wells is only "50-50" to make it to the post for the premier Classic at Epsom on Saturday.

He has lost his place at the head of the market to Snow Ridge, and is not even second favourite, with that honour going to North Light.

"Reliable sources are now saying that Yeats is only 50-50 to line-up and we haven't struck a bet on him for weeks," said Coral spokesman Simon Clare.

"We have massive liabilities on Yeats and it will be a major blow to ante-post punters if he fails to make the starting line-up."

Yeats's participation could hinge on he reacts to being exercised at Ballydoyle.

O'Brien said: "Yeats worked great yesterday, but the trouble is that his back was stiff again in the afternoon. We are still in the jungle but we are surviving.

"I'm hopeful that we will get there, but it's a day-by-day thing now."

Snow Ridge is now favourite to give Frankie Dettori a first success in the race he wants to win above all others.

The Diesis colt will be having his first outing for almost 10 months in the Group One contest at Epsom tomorrow, having been off the track since his two-length second to Falbrav in the Juddmonte International Stakes at York.

And Callaghan believes the four-year-old to have done sufficient work to go to post as fit as he needs to be.

"He looks very well and we've got to start somewhere. The intention is to run him on Friday," the Newmarket trainer said.

"Although he's been off for 10 months, I think he's done enough work. He's ready to run a really good race and I am not worried about either the track or the trip for him.

"If he pleases us at Epsom, I would think he would certainly run in the King George and it's hard to disagree that he's one of the better older horses around this year."

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Prix Lupin winner Voix du Nord, who according to his jockey Dominique Boeuf is "made of steel", heads a list of 28 horses left in the French Derby on Sunday.

The David Smaga-trained son of Valanour underlined his Prix du Jockey-Club claims with a thrilling nose verdict over Millemix in the Group One trial captured last year by Dalakhani en route to glory at Chantilly.